Field of the Invention
This invention relates to the production of carbohydrate partial esters by transesterification of glycoses with fatty acid esters in the presence of emulsifiers and a special catalyst mixture and to the use of the substances obtainable by this process.
Background and Related Art
Carbohydrate esters, which are often also referred to as “sugar esters”, are esters of mono- or oligosaccharides and, in a broader sense, of sugar alcohols with organic or inorganic acids. Carbohydrate esters have pronounced surface-active properties, so that they are now regarded as an independent class of compounds (so-called “sugar surfactants”). By virtue of their favorable dermatological and toxicological compatibility, carbohydrate esters are mainly used as emulsifiers for the production of food and cosmetic products. Sucrose polyesters with 6 to 8 fatty acid residues are used in the diets of overweight people as a fat substitute, as they have no value to the organism and, in addition, are said to bind LDL cholesterol in the stomach.
There are various known processes for the production of sugar esters. Normally, glycoses are subjected to a transesterification with fatty acid methyl esters in the presence of alkaline catalysts, and optionally emulsifiers, and in the presence or absence of solvents. The solvent-free variant is preferred, in particular, when the sugar esters are to be used in the food industry. For example, DE 4131505 describes a process for working up sucrose fatty acid esters, which have been produced by the solventless transesterification of sucrose with fatty acid alkyl ester in the presence of a basic transesterification catalyst, the unreacted sucrose being filtered off at a temperature between the melting points of the sucrose used and the sucrose ester produced, and unreacted fatty acid alkyl ester then being distilled off from the reaction mixture. EP 254376 relates to a solventless, complete process for the production of polyol fatty acid esters in which less than half the hydroxyl groups are esterified, a solvent optionally having to be added intermediately in order to form catalytically-active polyol anions. Solventless processes for the production of highly esterified polyol fatty acid esters are also the subject of WO 99/38875, EP 548272, EP 550526, EP 322971 and EP 349059.
A solventless process for the production of sucrose fatty acid esters in the presence of a basic catalyst is the subject of EP 885 898. In this process, sucrose is reacted with fatty acid alkyl esters at 120° to 160° C. under pressures of 25 to 100 mbar, after which the temperature is reduced to 90° to 130° C., and the resulting reaction product is filtered without addition of a solvent. This process provides dark brown, viscous crude products, which are bleached in a following step. A process for the production of polyol polyesters with improved color quality is the subject of EP 349221. In this process, C1-3-alkyl fatty acid esters, having a carbonyl content of less than 200 ppm, are reacted with a polyol in the presence of a catalyst and, at the same time, the alcohol formed is removed from the equilibrium.
DE 19717968 describes a process for the production of carbohydrate partial esters in which the alkali-catalyzed transesterification is carried out in the presence of emulsifiers and a catalytically-active system of alkali metal carbonates, with fatty acid lower alkyl esters being formed in a first step and then reacted with glycoses and carbohydrate partial esters as emulsifiers. This process also provides relatively strongly-colored crude products, which also have a high ash content (DGF Methoden “Asche C III, 1097). FR 9916213 describes mixtures based on 10 to 90%, by weight sugar esters and 90 to 10%, by weight hydrocarbons, which are used in cosmetic formulations. However, pelleted compositions of these mixtures secrete oil after brief storage.
The solventless processes normally provide a complex reaction mixture which, to a large extent, contains unreacted starting products that are difficult to remove. These products are often very dark in color. Lighter-colored products are obtained by processes, such as that described in EP 349221. However, these processes require prepurified fatty acid alkyl esters, requiring an additional reaction step.
The problem addressed by the present invention was to provide an alternative economical, solventless process with high yields and short reaction times, which would give crude products with improved color and a reduced ash content, and which would not require prepurification of the starting materials.
It has surprisingly been found that a significant process improvement may be achieved if a mixture of alkali metal carbonate and alkali metal hypophosphite is used as the catalyst mixture.